#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
class Trai
{
private:
    float a;
    float b;
    float c;
public:
    Trai(float e, float f, float g)
    {
        a = e; b = f; c = g;
    }
    float area()const
    {
        float p = (a + b + c) / 2;
        return (float)sqrt(p * (p - a) * (p - b) * (p - c));
    }
};
int main()
{
    float a; float b; float c;
    cin >> a >> b >> c;
    cout << fixed << setprecision(1) << Trai(a, b, c).area() << endl;
    return 0;
}